The present invention relates to an electric motor. More particularly, the invention relates to an electric motor for submersible drives involving high gas pressures in the interior of the motor.
The electric motor possesses a motor housing, preferably pressure-resistant, in which a stator is installed and a rotor is mounted for rotation. The stator consists of a stator lamination pack and a stator winding. A box for electrical connections is generally mounted on the pressure-resistant motor housing. The pressure-resistant motor housing is preferably filled with a gas, more particularly under high pressure. The gas pressure may be between 5 and 150 bar or even higher.
For plants operating under water, as for instance for pumping petroleum and natural gas, particularly in very deep water, electric motors are utilized, which in accordance with the particular drive concept are filled with a liquid or gaseous medium. The pressure-resistant motor housing of such drives is so designed that it is able to resist the pressure in very deep water.
An electric motor of the type mentioned is disclosed in the German patent publication 3,925,337 A 1.
A significant disadvantage of a gas-filled motor according to prior art is that the rotor and fan design for the respective cooling systems cause high frictional losses with the high gas pressure in the interior of the motor with the result that the dissipated heat can not be sufficiently conducted away.
The German patent publication 3,729,486 C1 discloses an electric motor which is accommodated with a compressor. The pressure-resistant housing and the motor are filled with gas in a common pressure-resistant housing. For cooling the motor the gas flows through the motor and an annular cooler or radiator surrounds the housing concentrically.
The European patent publication 0 297 274 and the prior German patent application P 41 00 135.4-32, refer to an electric motor, which together with the machine to be driven are mounted in a water-tight housing capsule. The pressure-resistant housing is filled with a gaseous or liquid medium. This filling surrounds the motor, which is filled with water or with oil.
Although the prior art motors are successful in certain fields of application, they are unsatisfactory with respect to the high frictional losses, and need for complex pressure compensating device and cooling systems.
The German patent 2,945,194 discloses an electric motor, wherein disks are secured to the rotor shaft, which rotate with the rotor shaft and are so arranged on the rotor shaft that between the part, which projects radially past the bore in the stator of the rotating disks and the end surfaces of the stator lamination pack there is in each case a narrow axial gap. Owing to the high speed of rotation of the rotor, the disks, when rotating, fling air out of the axial gap in the part of the disks which projects radially past the bore in the stator, such that the air becomes more rare in the hole in the stator. This causes a reduction in the air friction losses. The pumping action of the rotating disks is furthermore enhanced since pumping grooves are provided on the side, facing the stator lamination pack of the disks. In the case of the German patent 2,945,194 the rarefaction of the gas leads to a reduction in gas friction losses.
Similarly, the electric motor disclosed in the German patent 427,917 has reduced gas friction losses by rarefaction of the air.
The designs in accordance with the German patent 2,945,194 and the German patent 427,917 are however not applicable to motors operating with high internal gas pressures inside the housing.
In the German patent 2,928,393 a generator is disclosed which has an aluminum housing, in which a stator is attached and a rotor is mounted for rotating in relation to the stator. For the stator to be able to be held in position over a large operating temperature range and in order to provide for improved cooling, the stator has an external surface, which is in engagement with a press fit on an internal surface of the housing, over the entire operating range of the generator. In one of the surfaces a plurality of longitudinal grooves is formed so that the housing is able to be bent adjacent to the longitudinal grooves, when the generator is operating in the lower part of the temperature range. The longitudinal grooves are necessary in order for the electrical machine be utilized over the entire operating temperature range despite the use of materials with very different coefficients of thermal expansion. This design is however not suitable for high pressure motors.